The invention relates to thyristor-controlled AC/DC converters in general, and more particularly to the reduction of harmonics in such converters for the generation of a pure DC voltage at the output.
The invention is based on the fact that while harmonic components forming the ripple superimposed on the DC output voltage theoretically include as the lowest frequency harmonic a component of the order of f.sub.s .multidot.p (where p is the pulse number of the converter and f.sub.s the fundamental frequency of the AC power supply), due to unavoidable imperfections there also exist "non-canonical" harmonics which have frequencies lower than f.sub.s .multidot.p.
Conventional passive filtering techniques can handle the "canonical" harmonics of relatively high frequency, and this is achieved without too much difficulty or cost. Elimination of the "non-canonical" harmonics, however, which are of the order of f.sub.s, 2f.sub.s, 3f.sub.s, would require large and expensive passive filters to obtain sufficient attenuation at the lower frequencies of the "non-canonical" harmonics as well as at the higher frequencies of the "canonical" harmonics.
Accordingly, it is now proposed to use active filtering to reduce the amplitudes of the non-canonical low-frequency harmonics, thereby limiting the use of passive filters to harmonics of high frequency.
Typically, in a 12-pulse converter operating from the 60 Hz line, the non-canonical harmonics which are to be eliminated by active filtering would be 60 Hz, 120 Hz and 180 Hz, whereas the lowest order "canonical" harmonic would occur at (12.times.60)=720 Hz.
Non-dissipative, or "active" filtering is known. Active filtering for minimizing an undesired harmonic component through an active branch of current source has been done in the past by deriving from the output a control signal representative of the undesired component and injecting, through an additional power component controlled by the representative signal, into the output a compensating current of proper phase and magnitude to automatically neutralize the undesired component throughout the active branch. See, for instance, U.S. Pat. No. 4,241,395 of Eric J. Stacey et al.
In the proposed invention the thyristor converter itself acts as the active branch of current source, thus obviating the need of any additional power component to inject the compensating current.